This invention relates to auxiliary bass radiator units. That is to say, units which when installed in a loudspeaker enclosure form an auxiliary bass radiator.
Auxiliary bass radiators (ABRs) have been known for many years and are used to take the place of tuning ports in bass reflex loudspeaker enclosures.
A bass reflex loudspeaker enclosure, which is a fourth order acoustic system, has the bass output augmented by the Helmholtz resonance that occurs when the mass of the air in a tuning port resonates on the stiffness of the air inside the box. Generally, if such a system is designed properly, about half an octave of extra bass extension may be achieved by this means. The roll off rate for a fourth order, bass reflex. system is 24 dB per octave below the resonance frequency. For a closed loudspeaker enclosure with no tuning port, the rate is 12 dB per octave.
ABRs were devised to allow low frequency Helmholtz tunings to be achieved when physically small loudspeaker enclosures were needed. In such cases, the length of a non-ABR tuning port would be too long to be accommodated in the enclosure and would have troublesome pipe resonances of its own. An equivalent resonance frequency can, however, be achieved by replacing the mass of air in the tuning port with a solid mass on a separate suspension. Such an arrangement is effectively a loudspeaker diaphragm without either voice coil or magnet. By choosing the mass to match the diaphragm area and size of the enclosure, this mass spring resonator may be tuned to virtually any frequency desired in practice without the problems of length from which tuning ports suffered.
Usually, the free air resonance frequency of an ABR is made as low as possible and then, when it is placed in the enclosure, the stiffness of the air in the enclosure takes over as the major stiffness tuning factor. The combination of an ABR and enclosure has a response very similar to that which would have been achieved with a non-ABR tuning port.
If, however, the free air resonance is made too high, a dip occurs in the output of the system at the free air resonance frequency and the bass roll off rate is steeper above this free air resonance dip.
It is an object of the invention to provide an improved auxiliary bass radiator unit.
The present invention provides an auxiliary bass radiator unit for mounting in an aperture in a loudspeaker enclosure, the unit comprising a substantially rigid panel member mounted on a flexible surround member so as to allow the panel member, when mounted, in use, in the said aperture, to move in sympathy with sound waves within the enclosure, wherein the panel member is constrained to move as a hinged flap.
The invention is based on the realization that because ABRs had to be made to allow a long throw, that is a large movement back and forth, to occur at the mass spring or Helmholtz equivalent resonance frequency, they unfortunately allowed the ABR panel member to move in other modes than simply back and forth. For example, rocking and twisting modes were not prevented from occurring by the necessarily floppy suspension. These higher order modes coloured the resulting sound output from the speaker system.
In the construction according to the invention, the said higher order modes are substantially suppressed. The constraint substantially prevents rocking, twisting and other undesirable modes of vibration from occurring.
Advantageously, a hinge portion is included within the flexible surround member and mounts the panel member for movement as a hinged flap. Such a construction provides a simple way of constraining the movement of the panel member.
Advantageously, the outer periphery of the surround member is connected to a substantially rigid frame member for mounting the flexible surround member in the said aperture. Such a construction facilitates the mounting of the periphery of the surround member in the said aperture.
Preferably, the frame member includes a sealing bead of resilient material to seal the frame member in the said aperture. By this means, a good seal between the frame member and the aperture can readily be achieved.
Advantageously, the panel member includes a substantially straight side along at least part of which the hinge portion is provided. Such a construction is particularly simple.
The hinge portion may be defined by an integral part of the surround member. In particular, the hinge portion may comprise a flat web portion of the surround member.
Advantageously, the web portion is connected to an edge portion of the panel member and to a corresponding edge portion of the frame member and further includes an integral spur extending the flat web portion into a T-shaped cross-section, the spur being located between the edge of the panel member and the edge of the frame member. Such a construction provides a particularly effective form of hinge.
At locations other than in the hinge portion, the surround member may comprise a roll portion. The roll portion provides a simple means of mounting the free edges of the panel member.
Preferably, the roll portion tapers towards the hinge portion. By that means, greater movement at distances remote from the hinge portion is readily accommodated.
The roll portion may be of semi-circular cross-section.
Advantageously, the roll portion includes transverse ribs. By this means, any tendency of the surround member to pucker can be reduced.
Preferably, the ribs are formed by corrugations in the material of the surround member.
The panel member may have a plurality of substantially straight sides along one of which the hinged portion is provided.
The ribs may be provided at one or more corners, remote from the hinged portion, where two straight sides meet.
Advantageously, the said one or more corners are rounded. By this means also, any tendency to puckering can be reduced.
The panel member may be substantially rectangular with rounded corners at its end opposite to the hinged portion.
A short side of the rectangular panel member may have the hinged mounting.
The surround member may be made of resilient polymeric material, for example, a natural or synthetic rubber material.
The panel member may be made of plastics material, for example, polypropylene.
The frame member may be made of plastics material, for example, polypropylene.
The unit may be made by the co-injection moulding of component parts.
The panel member, when at rest, may be set back substantially from the surface of the frame member.
The panel member may have a thickness of at least 3 millimetres, or of at least 4 millimetres, or of at least 5 millimetres, or a thickness of between 4 and 10 millimetres. Such dimensions enable the panel member to have sufficient mass and stiffness to be used without the addition of a supplementary weighting or stiffening means.
The invention also provides a loudspeaker enclosure including an auxiliary bass radiator unit according to the invention.
Two such units may be provided on opposite sides of the enclosure. By this means, the physical stability of the enclosure when in use is improved.
The invention also provides a loudspeaker system comprising an enclosure as defined above and one or more loudspeaker drive units.